In recent years, in place of the conventional relief printing plate of lead produced by a casting machine, a relief printing plate of a resin is produced by providing a negative-type photosensitive resin layer, then irradiating it with light through a negative film to photocure selected portions of the photosensitive resin layer while leaving the other portions uncured and subsequently removing the uncured portions with an aqueous solvent or high air pressure. The resultant photocured resin relief plate is light in weight and ensures easy handling as compared to the lead relief plate, so that its use is rapidly being disseminated in the field of printing, particularly in the printing of newspapers.
Generally, the performance of the photocured resin relief plate is greatly affected by the hardness of an employed photocured resin layer. For example, in the case of a hard photocured resin relief plate, it has a drawback in that dimness occurs in printed images having a large area, such as large letter images, solid images and halftone images, because of poor ink transferability, although it is excellent in reproducibility of fine lines and small dot images in printing, because the degree of deformation of a relief top portion by a printing pressure is small. When a high printing pressure is employed in printing, in order to avoid the problem of the poor ink transferability mentioned above, another problem arises, such as the broadening of fine lines and small dot images in printing and lowering of printing resistance due to the abrasion of a relief top portion.
On the other hand, in the case of a soft photocured resin relief plate, printed images are likely to be broadened to thereby cause production of sharp prints to be difficult, because of the inclination of a relief top portion to deform by a printing pressure, and especially when use is made of a printing machine having poor mechanical precision, an extremely high printing pressure is locally exerted, so that it is difficult to prevent non-uniform printed image broadening although it is so excellent in ink transferability that satisfactory ink transferability is ensured even on a matter to be printed which has poor surface smoothness, such as woody paper and news printing paper.
To avoid these drawbacks, in a flexographic printing it was attempted to use a printing plate having a soft elastic layer disposed under a substrate. However, desirable improvement of printing performance was not attained by this method because the prevention of image broadening in printing was less satisfactory due to the cushion effect of the elastic layer occurring only through the substrate, and because dimness tended to occur in, for example, printed solid images due to the escape of a printing pressure as a result of the attachment of the cushion layer.
In Japanese Patent Application Laid-Open Specification No. 54-92402, a photocured resin relief plate was proposed which is comprised of two different photocured resin layers having a hardness difference of from 10.degree. to 20.degree. in Shore hardness. In this plate, the hardness of the relief top layer was rendered greater than the hardness of the relief lower layer so that the deformation of the top portion of the relief by a printing pressure was minimized. However, the thickness of the relief layer at a highlight portion was identical with the thickness of the relief layer at a portion other than the highlight portion, so that the image broadening, especially at a highlight portion having a dot percent of 7% or less by a printing pressure, was not satisfactorily suppressed to thereby cause production of a desired printed matter to be infeasible.
Further, in Japanese Patent Application Laid-Open Specification No. 55-6392 (corresponding to U.K. Patent Application Laid-Open Specification No. 2,024,441), a photocured resin relief plate was proposed, which is comprised of two different resin layers having a modulus difference of at least 50 psi after light exposure to light, which resin layers resulted from a photosensitive resin layer corresponding to a relief lower layer and a photosensitive resin layer corresponding to a relief top layer, the former photosensitive resin layer exhibiting a photopolymerization rate of 1.5 to 200 times that of the latter photosensitive resin layer. Still further, in Japanese Patent Application Laid-Open Specification No. 62-124559, a photosensitive resin plate for relief printing was proposed, which is comprised of two photosensitive resin layers, i.e., a photosensitive resin layer corresponding to a relief lower layer and a photosensitive resin layer corresponding to a relief top layer, the minimum insolubilization light exposure value of the former layer being 5 mJ/cm.sup.2 or less and up to 1/2 of that of the latter layer. By these proposals using two different photosensitive resin layers having such a photosensitivity difference, that a photosensitive resin layer corresponding to a relief top layer has less photosensitivity, the image size at the relief surface of the resultant printing plate can be rendered smaller than that of the negative film employed in the imagewise light exposure and printing performance can only be improved so much. However, according to these proposals, the relief thickness at a highlight portion is identical with that at a portion other than the highlight portion, so that the broadening of printed images by a printing pressure cannot be prevented and satisfactory printed matter cannot be obtained.
In the above situation, the present inventors have made extensive and intensive studies with a view toward developing a photocured resin relief plate having excellent image reproducibility and ensuring excellent printing performance. As a result, the present inventors have unexpectedly found that the thickness of the relief layer at a highlight portion having a dot percent of 7% or less can be rendered smaller than the thickness of the relief layer at a portion other than the highlight portion by exposing two different photosensitive resin layers having different photosensitive characteristics, which photosensitive characteristics satisfy a specific relationship, to light for a predetermined period of time to thereby form a relief base layer and a relief top layer, and that thereby, the broadening of printed images can be suppressed at the highlight portion. Based on this novel finding, the present invention has been completed.